This invention relates to reflective liquid crystal displays (R-LCDs), and more particularly relates to compensation for the residual retardation in the driven state of the liquid crystal (LC), as well as compensation for the so-called "skew-angle" effect.
R-LCDs are attractive for use as the light modulators in projection display systems, due in part to the ability to integrate the pixel switches and interconnecting circuitry and components on a silicon substrate, below the reflective layer, enabling the high pixel density needed for high resolution displays.
R-LCDs operating in the "twisted nematic" (TN) mode have orientation layers in contact with the LC layer with preferred directions of orientation usually set at an angle of from 30 to 90 degrees to one another. The nematic LC molecules next to these orientation layers tend to orient themselves with their long axes (directors) parallel to the orientation direction. This orientation distorts the normally mutually parallel relationship of the directors, forcing the directors in the bulk of the LC molecules between the oriented LC molecules to twist 45 degrees.
Plane-polarized light striking the transmissive side of the R-LCD normal to the plane of the LC layer and parallel to the director orientation, is rotated a number of degrees as it passes to the reflective side. In some modes, for example the 45 degree twist mode, upon reflection, the light is circularly polarized, and upon leaving the liquid crystal, the light has undergone a net rotation of 90 degrees.
Light modulation is achieved by the application of an electric field, under the influence of which the bulk of the LC molecules tend to untwist, to a degree dependent on the strength of the field. In the extreme untwisted state, the polarized light traverses the R-LCD with substantially no rotation. An analyzer such as a polarizing beam splitter (PBS) at the transmissive side of the R-LCD transmits more or less of the polarized light in accordance with its degree of rotation.
Unfortunately, the applied field, even in the extreme state, is insufficient to influence the LC molecules in regions next to the orientation layers. These so-called "dead layers" impart a residual birefringence or retardation to the R-LCD, which changes the plane polarized light to slightly elliptically polarized light, the minor component of which leaks through the analyzer, thus decreasing the contrast of the display.
U.S. patent application Ser. No. 09/097969 filed Jun. 16, 1998, and assigned to the present assignee, describes a compensator which compensates for the residual retardation of the R-LCD in the driven state. This compensator has a retardance (d.DELTA.n).sub.R of about 20-100 nm for the normally-white variant (normally-white means going from bright to dark as the driving voltage increases), and about 150-250 nm for the normally-black variant (normally-black means going from dark to bright as the driving voltage increases). The compensator is oriented with its optical axis between (80+.phi./2) degrees and (100+.phi.) degrees with the liquid crystal orientation at the transmissive side of the LCD, where .phi. is the twist angle. For .phi.=45 degrees, the range 102.5-145 degrees. Such an orientation results in the cancellation of the minor component of the elliptically polarized light exiting from the R-LCD.
In cases in which the illumination beam is not collimated, but is cone-shaped, for example, convergent from the source onto the R-LCD, and the analyzer is oriented at an angle to the polarizer, there will be light leakage through the analyzer, causing a reduction in contrast of the display. This leakage is known as the "skew-angle" effect. This effect is worse in the case of a polarizing beam splitter, in which the embedded analyzer is designed to have high transmittance and work well over a narrow range of cone angles.
U.S. Pat. No. 5,327,270 and PCT International publication WO 95/13561 address the skew-angle effect by introducing a second compensator in the form of a quarter wave plate oriented with its optical axis either parallel to or perpendicular to the plane of the incident polarization.